CVE-2025-38350

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

net/sched: Always pass notifications when child class becomes empty

Certain classful qdiscs may invoke their classes' dequeue handler on an

enqueue operation. This may unexpectedly empty the child qdisc and thus

make an in-flight class passive via qlen_notify(). Most qdiscs do not

expect such behaviour at this point in time and may re-activate the

class eventually anyways which will lead to a use-after-free.

The referenced fix commit attempted to fix this behavior for the HFSC

case by moving the backlog accounting around, though this turned out to

be incomplete since the parent's parent may run into the issue too.

The following reproducer demonstrates this use-after-free:

tc qdisc add dev lo root handle 1: drr

tc filter add dev lo parent 1: basic classid 1:1

tc class add dev lo parent 1: classid 1:1 drr

tc qdisc add dev lo parent 1:1 handle 2: hfsc def 1

tc class add dev lo parent 2: classid 2:1

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References